Packet transmissions over cellular radio

ABSTRACT

An improved system is achieved in a cellular arrangement where mobile units employ a moveable slot TDM approach to send packets to base stations, and where the base stations use a non-contention approach. When a base station transmits information packets to different mobile units, it merely queues the packets and transmits over a given channel. In a corresponding inbound channel, the mobile units transmit packets to the base station using the MSTDM protocol. The base station also transmits information regarding whether the inbound frequency is occupied by a signal that is being transmitted to the base station, or whether a collision exists. A collision on an inbound frequency occurs when more than one signal is simultaneously transmitted on the inbound frequency. On an additional control channel that is shared by all base stations, the base station sends information in a TDM fashion.

RELATED APPLICATIONS

[0001] This application is a continuation of application No. 08/982,450,filed Dec. 2, 1997.

[0002] This application is related to U.S. patent applications Ser. No.08/082,634 and Ser. No. 08/982,571 filed concurrently herewith, entitled“Packet Switching Architecture In Cellular Radio” and “Overload ControlIn A Packet-Switching Cellular Environment,” respectively.

BACKGROUND

[0003] This invention relates to cellular telephony and, moreparticularly, to the use of packet techniques in cellular telephony.

[0004] The number of people that use cellular telephones is continuallyincreasing.

[0005] Because the available bandwidth is controlled by governmentalregulations, providers of cellular telephony are meeting the increase inusers by establishing smaller cell sizes. Smaller cell sizes accommodatelarger numbers of mobile units within the same overall bandwidth becausesmaller cell sizes effectively increase the rate of bandwidth re-use perunit area. However, as cell sizes shrink, mobile units move betweencells more frequently. In a circuit switched system, each move requiresthat one circuit be torn down and another one set up. Consequently, ascell sizes decrease, the work associated with handing off users betweencells increases. In addition, when a mobile unit traverses more cellsduring its connection, it is more likely that the mobile unit willencounter a cell with more units than the bandwidth can support.

[0006] Packet switching, as compared to circuit switching, reduces thework required for hand off because addresses embedded within the packetsare used to route individual packets rather than setting up and tearingdown circuits. Packet switching was used in early military cellularsystems. Those networks were designed to be rapidly deployed, were aimedprimarily for wireless interconnection between mobile units, and werenot connected to a wired backbone network.

[0007] Currently, the prevalent commercial cellular system in the UnitedStates is a circuit switched arrangement that employs Time DivisionMultiplexing (TDM). Another system, which is also a circuit switchedsystem, employs Code Division Multiple Access (CDMA). These cellularsystems can transmit data in the form of packets, but that does notconstitute “packet switching,” either in the sense employed in theaforementioned military system or in the sense employed in thisdisclosure. Specifically, while the data may have a packet format, theswitching within the cellular environment is not based on the explicitaddress information in the packets. For example, in TDM the address isimplicit in the frequency and time slot at which the mobile unitoperates.

[0008] The explicit addressing characteristic of packet switching ismore flexible than implicit addressing. With explicit addressing, thecapacity on the shared medium can be reassigned as required and thedestination can be changed without advance notice. Because of that, itis beneficial to fashion a packet switching approach for cellularcommunication that interfaces effectively with a wired backbone network.

SUMMARY

[0009] An improved system is achieved in a cellular arrangement wheremobile units employ a moveable slot TDM approach to send packets to basestations, and where the base stations use a non-contention approach.More specifically, from among a specified band of frequencies, the basestation selects a frequency (an outbound frequency) and uses it totransmit information packets to a mobile unit within the cell covered bythe base station. The information packets are simply queued withinformation packets destined to other mobile units, as necessary, andtransmitted over the outbound frequency. Corresponding to the outboundfrequency there is an inbound frequency that is used by the mobile unitto transmit information packets to the base station.

[0010] The base station also transmits information about the status ofthe inbound frequency; i.e., information that informs whether theinbound frequency is occupied by a signal that is being transmitted tothe base station, or whether a collision exists. A collision on aninbound frequency occurs when more than one signal is simultaneouslytransmitted on the inbound frequency. This information can becommunicated over a separate channel. It can also be embedded within thestream of bits on the outbound frequency by appropriate injection ofData Link Escape (DLE) sequences.

[0011] In addition to transmitting information packets and embedded DLEsequences over the outbound frequencies, the base station transmitsgeneral “control-channel” information, in a TDM fashion, over afrequency that is shared by all base stations.

[0012] The mobile units, on the other hand, employ a movable slot TDM(MSTDM) protocol in their transmissions to a base station. Before ittransmits, each mobile unit listens to a signal from the base stationthat informs it of the status of the in-bound frequency that the mobileunit is assigned to use for transmissions to the base station. If theDLE sequences inform a mobile unit that the inbound frequency is notbusy (when such an approach is used), the mobile unit is permitted tobegin transmissions. When the mobile unit wishes to transmit a datapacket or a first voice packet, the mobile unit is also sensitive tocollision information (e.g., delivered via the DLE sequences). When acollision is detected, such a mobile unit stops transmitting and triesagain later. When the mobile unit wishes to transmit continuation voicepackets, it only listens for a non-busy inbound frequency before ittransmits. It does not stop transmitting in case of a collision.Continuation packets include a short header that carries no information,to insure against corruption of information in case of a collision.

[0013] With the use of MSTDM for inbound traffic, the re-use offrequencies is simplified in the system. Specifically, the onlyrequirement is that adjacent cells should not use the same frequencies.Accordingly, the available band of frequencies (minus the “controlchannel” frequency) is divided into three sub-bands, and differentsub-bands are assigned adjacent cells. The control channel is broken upinto time segments, and the time segments are assigned to adjacent basestations in the same way that the sub-bands are assigned.

BRIEF DESCRIPTION OF THE DRAWING

[0014]FIG. 1 presents a general block diagram of a system that employspacket switching and switching agents;

[0015]FIG. 2 presents details about an illustrative topology of thestructure between the base stations and the switching agents;

[0016]FIG. 3 shows frequency allocations for cells;

[0017]FIG. 4 shows the power considerations for transmissions by mobileunits;

[0018]FIG. 5 depicts the cell hysteresis that is created with properselection of base station and mobile unit transmission power; and

[0019]FIG. 6 illustrates a portion of a mobile unit's structure thatallows graceful degradation in case of overload conditions at a basestation.

DETAILED DESCRIPTION Structure

[0020]FIG. 1 depicts the general structure of a network that includes awired portion above dashed line 10 and a wireless portion below dashedline 10. The wireless network comprises cells, depicted in the form ofhexagons, e.g., hexagons 11-18, which completely cover a givencollection of service areas. A service area can span any convenientgeography, such as a city, a city and its suburbs, or an area into whichpeople frequently commute. In the center of each cell there is a basestation, e.g., element 21, that provides connection between the wirelessnetwork and the wired network. Lines 31-36 diagrammatically show thisconnection. Each switching center that is coupled to base stationsconnects the base stations with the existing wide-area communicationsnetwork 100—for instance, the conventional, circuit switched, telephonenetwork, or the Internet.

[0021] In accordance with the principles disclosed herein, the couplingbetween the base stations and the service areas on one side, and network100 on the other side, is effected through switching agents. Eachregistered mobile unit is represented by a switching agent at theinterface to network 100; for example, agents 61-64. The agent isresponsible for translating between the formats that are used in network100 and the service areas (if necessary), and for all operations neededfor mobility.

[0022] Service areas 40 and 50 are wired packet switched networks. Inaddition to other functions, they serve as buffers to remove allresponsibility for mobility from network 100. A service area can haveany one of a number of different topologies, such as the well-knownstar, tree, mesh, ring, bus, or regular mesh. All of these networks cansupport packet switching. Of course, a service area should employ atopology that allows for easy interconnection with adjacent serviceareas. Moreover, interconnected adjacent areas should be arranged tohave a hysteresis at the areas' boundaries. By hysteresis I mean thatinstead of moving a switching agent to an adjacent service area as soonas the mobile unit arrives at a cell of that adjacent service area, theswitching agent is kept in the original service area until the mobileunit moves more deeply into the adjacent service area. This hysteresisreduces processing associated with migration of a switching agent fromone service area to another, because at times the mobile unit returns tothe original service area. This hysteresis is depicted in FIG. 1 by theoverlap between service areas 40 and 50.

[0023] The specific architecture, or topology, of the service areas isnot important to the broad principles of this invention; but for sake ofcompleteness, it is useful to review the various topologies that areimplementable.

[0024] One such topology is the star topology, where lines from all ofthe base stations in a service area terminate at one packet switch (the“central office”). The switching agents are installed between the“central office” of each service area and a switch on the wide-areacommunications infrastructure 100, say in that “central office”. The“central offices” of the various service areas are advantageouslyinterconnected to allow for easy migration of switching agents, e.g. viaseveral lines running between them.

[0025] As a mobile unit moves between cells within a service area, theconnection through the central office packet switch changes, but theconnection on infrastructure 100 remains fixed. A disadvantage of thistopology is that there is no redundancy in the connection between thebase stations and the central office.

[0026] A tree topology is similar to a CATV network, when the “centraloffice” is located at the root, and the cells are located at the leavesof branches. For packets destined to the base stations, routingdecisions are made at each branch split in the tree. For trafficdestined to the head end, multiplexers combine the packets and send themtoward the “central office”. An advantage of this approach is that theCATV infrastructure is in place in most parts of the United States, andpacket multiplexers and splitters are commercially available. Theoverlap between service areas can be created by placing asplitter/multiplexer at the trunk of the tree and using the multiplexerto switch a number of connections to an adjacent tree. The principledisadvantage of this architecture is its weak reliability. There aremany locations where the failure of a single line or component candisrupt communications for a large number of cells.

[0027] A general mesh topology can be implemented by a network ofInternet routers between the central office and the base stations. Thistype of network can be made as reliable as needed by installingredundant lines and routers. Service areas can be interconnected throughthe routers. The disadvantage of this approach is the expense oflocating a router at each cell site. Routers may be used advantageouslywithin the service areas, but a simpler device should be associated witheach cell.

[0028] Two possible distribution networks that are considerably simplerthan routers are the FDDI dual ring network, and the DQDB dual busnetwork. Both of these networks can survive single failures. Thedisadvantages of these networks are that it is difficult to interconnectthem to create overlapping service areas, and the load per linkincreases linearly with the number of nodes in the network. The lattercharacteristic constrains the number of base stations that can belocated on the same network.

[0029] The two disadvantages associated with FDDI and DQDB networks areovercome by another regular topology, the Manhattan Street Network(MSN), which was disclosed by me in U.S. Pat. No. 4,797,882, issued Jan.10, 1989. Regular arrays of MSN's can be interconnected into largerregular arrays to construct overlapping service areas. The MSN can alsobe partitioned into non-interfering, independent, communities ofinterest, which makes it possible to support arbitrarily large numbersof base stations that do not communicate with one another.

[0030] Actually, the disadvantages associated with specific technologiesare eliminated in the arrangement shown in FIG. 2 by combining severaltechnologies. In FIG. 2, the “central office” switches in each servicearea, such as switches 41, 51, 61, and 71, concentrate the connectionsfrom network 100 to a router. For example, switch 41 has a number oflogical connections to switch 101 on one side (by means of the variousswitching agents) and a physical connection to router 81 on the otherside. In the reverse direction, switch 41 fans out the connections fromthe router to switch 101. Router 81 is but one of a number of routersthat make up router network 80. Several service areas (and theirassociated “central office” switches) are connected to router network80, and several other connections couple network 80 to distributionnetwork 90. Those connections couple network 80 to neighborhoods ofnetwork 90, such as neighborhoods 95 and 96. The connection to eachneighborhood is, advantageously, a multiple connection. This eliminatesthe problem of a single point of failure. Additionally, the routerswithin network 80 are multiply interconnected for increased connectivityand reliability. The distribution network in FIG. 2 is an MSN networkand, as indicated above, it comprises neighborhoods. A service area canhave several neighborhoods. As an aside, the functionality of network 90can subsume that of network 80. Network 80 is depicted to illustrate thefact that different networks can be employed. Indeed, currently thecomponents that make up network 80 are readily available commercially,and the use of network 80 allows network 90 to be smaller.

General Operation

[0031] The operation of the FIG. 2 network is quite effective. Eachmobile unit that is known to be present in the area (i.e., isregistered) has an associated switching agent—which is a softwaremodule, or object—at a gateway between a service area and network 100.For convenience, the switching agent resides in a node within a servicearea, and this disclosure refers to this node as a “central office”.Information that needs to be sent by network 100 to a particular mobileunit is transferred to that unit's switching agent. From the switchingagent, packets are sent to the mobile unit via a path that comprises theservice area's “central office” where the switching agent resides, oneor more routers in network 80, and one or more nodes in network 90.Packets that emanate from a particular mobile unit are aimed at itsassociated switching agent. That is, they conveniently contain anaddress that identifies the “central office” and the switching agent.They also contain the address of the base station that is to receive themobile unit's packets. The latter address allows the “central office” todecide whether to migrate the switching agent to another “centraloffice” (thereby realizing the service area hysteresis disclosed above).From the base station, the packets enter an MSN (for example, MSN 95)and then they are passed to a router within network 80, e.g., router 85.Network 80 routes the packets to the switch with which the mobile unit'sswitching agent associates (e.g., switch 41). All this is done based onthe addresses contained in the packets.

[0032] Typically, when the mobile unit moves to an adjacent cell, thereis no effect on operation other than the fact that the packet enters theMSN network (e.g., network 95) at a different point. On occasion,however, when the mobile unit moves from a cell in one service area intoa cell in an adjacent service area (and not into an area where the twoservice areas overlap), the operation does change. Specifically, the“central office” realizes that the base station, which sent the packets,is far removed from the geographical area that is normally handled bythe “central office” and the central office accordingly migrates theswitching agent to a new “central office”.

[0033] In such an event, the connection with network 100 also needs tobe changed because network 100 needs to communicate with the switchingagent at its new location. When network 100 is a circuit switchednetwork, the existing circuit to the switching agent needs to be torndown, and a new circuit needs to be established pointing to the servicearea to which the switching agent was moved. When network 100 is apacket network, e.g., Internet, then the accounting for the movedswitching agent must be carried out with whatever particular protocol isemployed in the network.

[0034] When a mobile unit wishes to register itself, it transmits apacket without identifying a destination switching agent. The basestation accepts the packet and routes it to a central office that isassigned to the base station. That is, the base station directs thepackets to a “central office” onto which it homes. Since the packet doesnot identify a destination switching agent, the central office createsone (after appropriate service provision tests have been met) andresponds to the mobile unit with the switching agent's identity. When amobile unit wishes to initiate a call, it sends a control packet thatcauses the switching agent to appropriately engage network 100 toestablish the desired connection.

[0035] When a call is initiated to a mobile unit that is not registered,there is no switching agent available, and the calling party receives amessage to the effect that the mobile unit is not found. When a call isinitiated to a mobile unit that is inactive, albeit registered andhaving an agent, the agent can establish contact with the inactivemobile unit over a common control channel.

[0036] Once a contact is established with a mobile unit, the switchingagent sends out encapsulated packets (i.e., each being a packet within apacket) to the mobile unit. The outer packet is addressed to aparticular cell, or base station, while the inner packet is addressed toa particular mobile unit. The agent needs to change only the address ofthe outer packet when the mobile unit moves from cell to cell. But, thatis a lot less work—one bookkeeping operation—than setting up and tearingdown a circuit-switched connection.

Communication to the Base Station

[0037] Current cellular systems are basically circuit switched systems.Such systems inherently dedicate a channel to a particular call, and thecapacity of that channel is captured by that call whether or not thatcall actually utilizes the captured capacity. In a sense, this is aninefficiency. The FIGS. 1 and 2 systems are packet systems that userandom access techniques. Random access techniques do not inherentlyassign a particular capacity to a call and therefore have the potentialfor a more effective utilization of the available bandwidth. However,pure random access techniques—where a mobile unit is allowed to transmitat will—also possess a characteristic that causes inefficiency.Specifically, there is clearly a potential for collision of data whentwo or more units are transmitting at the same time. Some capacity isused by virtue of the means that are provided to resolve contention overuse of the inbound channel, and whatever capacity is so used constitutesinefficiency.

[0038] What is interesting about cellular networks as they aredeveloping is the fact that they are shrinking in size. One consequenceof the shrinking size is a smaller propagation delay within a cell. Thesmaller propagation delay makes it possible to use efficient contentiondetection strategies, such as the Carrier Sense Multiple Access (CSMA)protocol or CSMA with collision detection (CSMA/CD). The latter is theprotocol that is used on the Ethernet.

[0039] In the CSMA protocol, a mobile unit listens to thetransmit-frequency before starting to transmit to determine whetheranother mobile unit is already using the channel. When the channel isnot busy, the mobile unit stops listening and starts transmitting.Because of propagation delays, however, it is possible for differentmobile units to find the channel not busy, to start transmitting, and tothus create a collision condition. CSMA/CD overcomes this problem bycontinuing to listen to the channel even after the mobile unit beginstransmitting. Collision is detected by the mobile unit when it finds outthat the channel is carrying more than just its own transmission. When acollision is detected, the unit stops transmitting, and tries toretransmit at another (randomly selected) time. Another unit that causedthe collision also stops its transmission and also retries to retransmitat a later time.

[0040] Identifying the presence of a collision condition requires that aunit detect the presence of a signal from another unit while ittransmits on the same frequency and thus also receives its own signal.In a cable environment, that is not too difficult because a properlyterminated cable does not produce echoes and therefore the cable unitcan easily subtract its own signal from the received signal. Even whenechoes exist, they are generally of small magnitude and relativelyconstant with time, allowing conventional echo cancellation techniquesto be used effectively. Collision detection in a radio network, however,is much more difficult because unexpected echoes (reflections) can bemuch stronger than the signal from other stations. Compensating forreflections in a wireless system requires considerable processing anddelay. This is particularly true in a mobile environment where theechoes change as a mobile unit moves from one location to another.

[0041] Contention systems can be used for voice communications. Howeverwith such use it is difficult to provide the required service guarantee.There are hybrid schemes that assign a channel after a user successfullycompletes a contention protocol—for example, “demand assigned multipleaccess” and “movable boundary” protocols. These systems require both acontention and a circuit allocation protocol.

[0042] A third alternative, which is a variant on CSMA/CD, is themoveable slot TDM (MSTDM) protocol. In MSTDM, sources also contend for achannel and then have a guaranteed rate until they relinquish thechannel. However, the MSTDM protocol is completely distributed, and theassigned channels as well as the random access packets use the sameprotocol to share all of the bandwidth.

[0043] In MSTDM, the notion is that there are data sources and voicesources. Data sources always use CSMA/CD. The voice sources use CSMA/CDonly for the first packet of information, and use CSMA for continuationpackets. A continuation packet is transmitted a fixed period after thesuccessful transmission of the previous packet in the same packetstream. If the channel is busy (e.g., because a data source grabbed thechannel a moment earlier), the continuation source waits and transmitsas soon as the channel becomes available. The CSMA protocol is viablefor continuation packets because the continuation voice packet includesa preempt signal at the beginning of the packet. Consequently, a datasource which sees a non-busy channel, starts transmitting and thendetects a collision condition can stop transmitting before it interfereswith the voice source.

[0044] The length of data packets is constrained to be shorter thancontinuation voice packets, so that a random access packet cannot forcea continuation voice packet that is waiting for the channel to collidewith the next scheduled voice packet. When a continuation voice packetis delayed, all of the samples that arrive while it is waiting areincluded in the packet. The next packet is scheduled a standard delayafter the channel is successfully acquired, rather than after thechannel should have been acquired. With this protocol, voice sourcesnever collide with each other, even when the channel utilization factorapproaches one. Therefore, there is no distortion of the voice sourceand the only voice delay is the packet assembly time.

[0045] While the MSTDM protocol allows mobile units that transmit voiceto operate mostly without the need to detect collisions, there is stillsome collision detection that must be carried out (for data packets andfor the first voice packet). As mentioned above, however, collisiondetection in a wireless environment is difficult because of the echoesproblem. I realized, however, that a two-channel approach can be adoptedfor cellular transmission which obviates the echoes problem, providesfor easy detection of collisions, and provides other benefits.

[0046] Specifically, in the two-channel collision detection approach themobile units send signals over one channel, and the base stationretransmits its received signal over another channel. By performing theretransmission over a channel that is non-interfering with the channelover which the mobile units transmit to the base stations, e.g. over adifferent frequency, avoidance of the echo problem becomes relativelyeasy. What the mobile units receive over the second channel is preciselywhat the base station has received. The strong echoes back to atransmitting mobile unit are simply not seen on that second channel. Bylistening to the “busy channel” over which the base station retransmitsits received signal, the mobile units can perform collision detectionand stop transmitting when appropriate.

[0047] Once a two-channel approach to collision detection is settledupon, one can observe that the two-channel approach allows the mobileunits to transmit signals only as far as the base station in the centerof the cell, i.e., the longest distance is the radius of the cell. Bycomparison, when the mobile units need to listen to transmissions ofother mobile units, the transmitted signal must be capable of reachingfrom one point on the circumference of the cell to a diametricallyopposite point on the circumference of the cell. This allows for agreater re-use of frequencies. FIG. 3 shows the single-channel approachon the left, and it requires seven different frequency bands. Bycomparison, the two-channel approach is shown on the right, and itrequires only three different frequencies. In this arrangement, allcells (hexagons) that are adjacent to a cell “A” have a differentfrequency from the frequency of cell “A”. The frequency of cell “A” isrepeated at cells whose centers are removed from the center of cell “A”by a distance of 3D{square root}{square root over (3/2)} distance units,where 2D is the distance between the center of cell “A” and any adjacentcell.

[0048]FIG. 4 shows still another benefit of the two-channel approach.Cell 201 uses frequency F1, cells 202, 204, and 206 use frequency F2,and cells 203, 205, and 207 use frequency F3. Cell 208 re-uses frequencyF1, and so the pattern repeats. A mobile unit at the edge of cell 201and communicating with the base station at the center of cell 201 needsto transmit with only enough power to reach the center of cell 201. Thisis depicted by circle 211 that is centered about mobile unit 210. Withthat in mind, one might realize that mobile unit 210 can transmit withsubstantially more power before its signal would reach the center ofcell 208 and interfere with the operation of that cell. Specifically, itcan transmit with power that approaches the coverage of circle 212. Ofcourse, one would not want to operate this way with no guard area, butit does suggest that both the power of the base station's transmitterand the power of the mobile units may be increased. Another way to viewit is that the cell sizes may be increased while keeping their centersconstant. Such an arrangement creates overlapping, non-interfering,cells, as shown in FIG. 5. The effect of allowing the size of the cellsto increase is dramatic. The area that is blank within hexagon 201 ofFIG. 5 is serviced by one of the three frequency assignments. The areasthat are striped are serviced by two frequencies (in the group ofthree), and the areas that are crosshatched can use all threefrequencies. In effect, the FIG.5 arrangement represents a plannedhysteresis in the cells.

Communication from the Base Station

[0049] The base station communicates with the mobile units on threelevels: it transmits information from network 100, it transmits “busychannel” information (for the MSTDM protocol), and it outputs othercontrol information over a control channel.

[0050] The outbound traffic of network 100 allows for a very simple airinterface. Since the base station is the sole signal source and there isno question of collisions or interference, packets destined to a numberof mobile units are assigned a frequency, queued as they arrive, andpromptly transmitted over that frequency. One needs to be concerned, ofcourse, with voice sources, where information must be sent at relativelyregular intervals. That concern has been put to rest in the prior artthrough use of appropriate voice encoding and scheduling techniques,which can be applied herein.

[0051] The information about the channel being busy or the channelexperiencing a collision can be sent over a separate channel, but itdoes not need to be. The base station can easily differentiate between achannel (i.e., a receiving frequency) being busy or not busy. Thatinformation can be imparted by the base station simply by transmittinginformation wherever the channel changes state.

[0052] Another way for the mobile units to receive the neededinformation is for the base station to send information at the instanceswhen the channel becomes busy with voice packets or with data packets.Since the length of the packets is known, the intervals when the channelis not busy can be ascertained by the mobile units themselves. Thus, theinformation that needs to be sent by the base station over the secondchannel of the two-channel collision detection approach requires verylittle capacity.

[0053] In addition to sending information that allows the mobile unitsto determine when the channel is not busy, information needs to be sentwhenever a collision occurs. The latter will occur fairly rarely insmall cells, but it still can happen. What is important in MSTDM is todetect collision with voice packets, because transmission ofcontinuation voice packets should not be aborted. Since data packets areaborted when a collision occurs, it is less important to detectcollisions early. In fact, collision for data packets can be detected bya base station when, after the packet is received, the packet's errordetection code indicates a reception error. Although some capacity inthe inbound channel could have been saved by having an early detectionof collision, the overall loss in capacity caused by employing aseparate channel for re-transmitting to the mobile units the signalreceived by the base station is not called for, in light of the lowprobability of collisions in small cells.

[0054] As indicated above, however, it is important to detect collisionbetween data packets and voice packets as early as possible. This may beachieved by incorporating a distinguishing feature in the packetsthemselves; e.g. a given bit is 0 for voice, and 1 for data.Alternatively, the distinguishing feature can be in the mode oftransmission that is employed. For example, data packets can betransmitted by mobile units with a suppressed carrier modulation scheme,whereas voice packets can be transmitted with a non-suppressed carriermodulation scheme.

[0055] Transmitting busy/not busy/collision information in the mannerdescribed above represents a very small amount of information and,therefore, in the FIGS. 1 and 2 systems this information is injectedinto the channel that carries the outbound information packets. This isachieved by the base station injecting a Data Link Escape character(DLE) into the bit stream followed by two information bits, as shown byway of example in the table below. Bits following the DLE Option IOption II 00 Channel became not busy Channel became busy with voice 01Channel became busy Channel became busy with data 10 A collision hasoccurred A collision has occurred

[0056] In addition to the channel that transmits outgoing informationpackets to the mobile units, each base station employs a common controlchannel for sending control information to the mobile units. Actually,since the amount of information that this channel needs to carry is notgreat, all base stations employ a common frequency for suchtransmissions. In order to avoid interference between adjacent basestations, each base station is assigned its own time slot on thatfrequency in such a way that base stations that might interfere with oneanother do not transmit at the same time. The interference between basestations using the control channel in the time domain has the sameconstraints as the interference in the frequency domain forcommunications from the base station to the mobile units. Therefore, thepattern for re-using time slots is the same as the pattern for re-usingfrequencies. For example, in the arrangement of FIG. 4 time is dividedinto three slots.

[0057] During its time slot a base station transmits a packetcontaining:

[0058] the base station's identity,

[0059] the set of transmit and receive channels that it is using,

[0060] its channel utilization,

[0061] broadcast requests from switching agents that are trying tolocate and activate mobile units, and

[0062] the list of mobile units that are currently registered to receivepackets in this cell and their allowed transmission rates (e.g., allpackets or only high-priority packets).

[0063] A packet on the control channel has a maximum length that isconstrained by the width of a time slot. If the complete list ofregistered mobile units cannot be transmitted in one packet, it iscontinued in the next packet, with an end-of-list identifier to indicatewhen the list is complete.

[0064] The power transmitted in the control channel is sufficient toguarantee that a mobile unit will always receive the signal from atleast one base station, but that power is lower than the power used inthe other channels. The power difference guarantees that a mobile unitcan receive data from any base station from which it receives a controlsignal.

[0065] A mobile unit joins the list of active stations in a cell bytransmitting a data packet to a base station whose signal it receives onthe control channel. The base station notifies the mobile unit'sswitching agent that all communications with the mobile unit are to beaddressed through this base station. If a mobile unit receives a controlsignal from several base stations, it can elect to join the base stationwith the lower utilization.

[0066] Mobile units are removed from the list of registered units in acell whenever they leave the cell, stop transmitting, or becomedisabled. Since a mobile unit cannot always notify the base station whencommunication has ended, the list is maintained in the base station assoft states. When the base station does not receive a packet from amobile unit for a period of time, the mobile unit is removed from thelist of registered units.

[0067] If a registered but inactive mobile unit receives its identifierin the broadcast segment of the control slot, it means that its agent istrying to establish a connection. If an active mobile unit receives itsidentifier in this segment, it means that its connection has beenbroken. In either instance, the mobile unit sends a data packet to thebase station in order to establish (or re-establish) a connection.

[0068] To summarize the air interface between a base station and mobileunits, the base station has a band of frequencies that it uses totransmit information packets to mobile units within the cell. Theinformation packets are simply queued as necessary and transmitted overthe base station transmit frequencies (outbound frequencies).Corresponding to each outbound frequency there is a frequency that isused by the mobile units to transmit information packets to the basestation (inbound frequency). Embedded within the stream of bits on theoutbound frequency which the base station transmits are DLE sequencesthat inform the mobile unit of the status of the inbound frequency. Inaddition, the base station transmits control channel information, in aTDM fashion, over a frequency that is shared by all base stations.

[0069] The mobile units, on the other hand, employ MSTDM protocol. Eachmobile unit listens before it transmits. If the DLE sequences inform amobile unit that the inbound frequency is not busy, the mobile unit ispermitted to begin transmissions. When the mobile unit wishes totransmit a data packet or a first voice packet, the mobile unit is alsosensitive to collision information delivered via the DLE sequences. Whena collision is detected, such a mobile unit stops transmitting and triesagain later. When the mobile unit wishes to transmit continuation voicepackets, it only listens for a non-busy inbound frequency before ittransmits. It does not stop transmitting in case of a collision.Continuation packets include a short header that carries no information,to insure against corruption of information in case of a collision.

Overload

[0070] The FIGS. 1 and 2 arrangement does not insure against overloadconditions. When a cell is heavily loaded, i.e., the inbound channel orthe outbound channel is close to being fully loaded most of the time,service can be denied to a new mobile unit that wishes to become active.However, that does not prevent overload conditions because an activemobile unit can move into the heavily loaded cell and cause an overloadcondition.

[0071] I realized that overload conditions can be accommodated with theMSTDM protocol mostly without denying access to mobile units. It helpswhen all of the periodic sources use the same packet rate. Instead oftreating a voice source as a single packet stream, the voice source canbe partitioned into two or more periodic streams. For instance, onestream can contain the samples that are needed for intelligiblecommunications, while the second stream can contain the samples thatprovide a higher quality connection. Normally, a mobile unit acquirestwo periodic channels and sends both packet streams. However, during anoverload the mobile unit can be instructed to only send one stream. Thecontrol channel provides the mechanism for notifying the mobile units ofhow many voice streams they may transmit. Of course, capacity in a cellcan also be increased for voice transmissions by asking the mobile unitsto transmit only during the active speech intervals in a TASI type ofoperation. TASI does tend to cut off a beginning portion of an activespeech interval. In this mode, therefore, when there are more activespeakers than available capacity, the beginning of active intervals maybe lost.

[0072] Another way to handle overload is to take advantage of thehysterisis in the cells. As shown in FIG. 5, there can be substantialareas within each cell that can be serviced by one or two other adjacentcells. Taking advantage of this hysteresis is applicable to bothoverload from active mobile units that come in (and stay) in the cell aswell as overload from inactive units wishing to become active. Themobile unit selects the base station with the strongest signal, which isnot over-utilized, and directs its packets to its switching agent viathe selected base station. The switching agent detects the identity ofthe base station from which the packets come and accordingly adjusts theaddress of the packets which the base station transmits when it wants tocommunicate information to the mobile unit.

[0073] A combination of the above techniques is also possible. Theprimary base station may constrain the mobile unit to send only packetsthat are needed for comprehensive speech, and the mobile unit may stillbe able to transmit the packets that can be used for higher qualitythrough another base station. In this instance, some of the packetswould arrive at the mobile unit's switching agent through one basestation and the remainder of the packets would arrive at the mobileunit's switching agent through another base station. The packets includea sequence number, if necessary, and the agent is responsible forproperly sequencing and spacing the packets.

[0074] During severe overload, a protocol is needed to redistributemobile units. A hybrid protocol that couples independent operations ofthe mobile units with the cooperative operations of the base stationsprovides a means to be both responsive to short term fluctuations and tolevel load imbalances over a large area. The mobile unit can quicklyshift its own load between overlapping cells, while base stations mustcooperate to redistribute the load over a wider area.

[0075] The protocol to move mobile units can use different types ofinformation. A simple protocol could allow a heavily utilized basestation to use the control channel to move some mobile units tooverlapping, less heavily utilized cells. The base stations in theadjacent cells could then move other mobile units to cells that arefurther from the congested cell, making it possible for the congestedcell to move more units. In a more sophisticated protocol, a basestation could take into account the number of units that adjacent cellscan redistribute and any other congested regions that may be near theadjacent cells.

[0076] With packet switching there is a possibility that packets arriveout of order and that the inter-packet timing will not be maintained,especially as a mobile unit changes base stations. To overcome thispotential problem, the packets in the arrangement disclosed hereincontain a sequence number and timing information so that the switchingagent can accurately reconstruct the signal before transmitting it tothe switched network. The RTP protocol, used on the Internet, includesthe necessary information.

[0077]FIG. 6 presents a general block diagram showing those portions ofa mobile unit that provide the capability to transmit packets asdescribed above. Receiver 304 receives signals from the base station andderives from the control channel information about overload. Thisinformation is applied to filter 300, coder 301, and transmitter 303.The voice signals are applied to filter 300, and appropriate signals aredeveloped at the output of filter 300 and applied to coders 301 and 302.Specifically, when a no-overload condition is indicated, coder 301receives the applied voice signal, and coder 301 develops a stream ofpackets corresponding to the applied voice signal. When an overloadcondition is indicated, coder 301 receives only a portion of the voicesignal that is needed for intelligibility, and coder 301 develops astream of packets at a rate that is lower than the rate developed for ano-overload condition. In system applications where a mobile unit isdirected to send some of its voice packets to a different base station(when there is an overload at the base station with which the mobileunit is communicating), transmitter 303 utilizes the output packetstream of coder 302. Accordingly, coder 302 is adapted to provide apacket stream in response to a signal that is developed by filter 300.The signal developed by filter 300 and applied to coder 302 is thatportion of the applied voice signal that complements the signal appliedto coder 301 when an overload condition exists. Illustratively, undernormal conditions, filter 300 merely applies its incoming speech signalto coder 301. When a control signal directs modified operation, filter300 separates the voice signal into a primary band and a secondary band.Both are shifted to base-band, and then applied to coders 301 and 302.

[0078] It may be noted that the overlap depicted in FIG. 5, whichprovides for cell hysteresis can be employed to advantage in more thanjust overload situations. For example, cell hysteresis eliminates thesometimes-occurring glitch in speech that comes about from cellswitching in the middle of an active speech interval. Cell hysteresisallows a moving mobile unit to stay in contact with the base station ofthe cell it has temporarily left, so that when the moving unit returnsto the cell, the process of moving to a different base station andreturning to the original base station is eliminated. Lastly, cellhysteresis reduces the surface area that loses service when a basestation fails.

The Switching Agent

[0079] The switching agent must translate between the data format thatis used on network 100 and the packet format. For example, when network100 carries speech in 64 Kbps (i.e., 8 bit samples are transmitted atthe rate of 8000 samples per second) and the packets carry 20 msec ofspeech each, the switching agent needs to assemble 20 msec worth ofspeech from network 100 in order to create a voice packet. In the otherdirection, the switching agent needs to take the 20 msec of speechdelivered by a packet, create samples, and evenly transmit them tonetwork 100.

[0080] The switching agent also keeps track of the base station that cantransmit to its mobile unit. As a mobile unit moves from cell to cell itnotifies its agent. When an agent must locate an inactive mobile unit,to place a phone call or to locate an active unit that has lost contact,it broadcasts a message to all of the base stations which is placed ontheir control channels. The hailed mobile unit responds and therebyinforms its switching agents of its whereabouts.

[0081] The switching agent also maintains a connection on network 100 onbehalf of the mobile unit. The agent breaks the connection at the end ofa communication session or when a failure occurs. Since the switchingagent is not always notified of a failure, it maintains a soft stateconnection so that resources in network 100 are not tied upindefinitely. If the switching agent stops receiving packets for aperiod of time, it first tries to contact the last base station, andthen tries a broadcast message to the mobile unit. If communication withthe mobile unit cannot be re-established, the connection on network 100is terminated.

[0082] For sake of completeness, it should be mentioned that the FIGS. 1and 2 arrangements do not, in and of themselves, overcome the well-knownprivacy problem of cellular telephony. Packets that are addressed to onemobile unit can be detected by another mobile unit. The advantage of thepacket system is that the data is digital and can be encrypted orscrambled more easily. In other words, the privacy problem is easilyovercome with the disclosed system by employing known encryptiontechniques, such as the one disclosed by Reeds et al in U.S. Pat. No.5,172,414, issued Dec. 15, 1992.

I claim:
 1. A method for communicating comprising the steps of: a mobileunit that wishes to transmit information in the form of packets, over afirst wireless, shared, inbound channel to a non-mobile base station,determines, based on information derived from a second wireless channelthat is other than said first wireless channel and logically distinctfrom a outbound channel by which said mobile unit receives informationpackets from said non-mobile base, whether the first wireless channel isbusy, when the first wireless channel is busy, the mobile unit refrainsfrom transmitting, and when the first wireless channel is not busy, themobile unit transmits a packet of information.
 2. The method of claim 1further comprising the steps of: when said packet is a data packet, themobile unit also determines, based on information from said secondwireless channel, whether a collision has occurred with a transmissionof a packet by some other mobile unit, when said packet is a data packetand the mobile unit determines the existence of said collision, themobile unit stops transmitting said packet, when said packet belongs toa voice source and it is a first packet of said voice source, the mobileunit determines, based on information from said second wireless channel,whether a collision has occurred with a transmission of a packet by someother mobile unit, when said packet belongs to said voice source and itis a first packet of said voice source, and the mobile unit determinesthe existence of said collision, the mobile unit stops transmitting saidpacket, and when said packet belongs to said voice source and it isother than said first packet of said voice source, the mobile unittransmits the entire packet without determining whether a collision hasoccurred with a transmission of a packet by some other mobile unit. 3.The method of claim 1 where said information from said second wirelesschannel is placed on said second wireless channel by said base station.4. The method of claim 3 where said second wireless channel is embeddedin said outbound channel.
 5. The method of claim 4 where saidinformation packets sent to said mobile units are data packets, voicepackets, or a mixture of both.
 6. The method of claim 3 where saidinformation which enables said mobile unit to determine whether saidfirst wireless channel is busy, comprises a first marker which indicatesthat said first wireless channel becomes busy, and a second marker whichindicates that said first wireless channel becomes not busy.
 7. Themethod of claim 3 where said information which enables said mobile unitto determine whether said first wireless channel is busy, comprises afirst marker which indicates that said first wireless channel becomesbusy with a data packet, and a second marker which indicates that saidfirst wireless channel becomes busy with a voice packet.
 8. The methodof claims 6 or 7 where said information which enables said mobile unitto determine the existence of a collision condition is a third marker.9. The method of claim 8 where said markers comprise a data link escapecharacter followed by two bits of information that are injected into awireless channel that said base station employs for sending informationpackets to said mobile unit.
 10. The method of claim 2 where said mobileunit transmits said data packet in one form of modulation, and transmitspackets of said voice source in another form of modulation.
 11. Themethod of claim 10 where said one form of modulation is a suppressedcarrier form of modulation, and said another form of modulation is anon-suppressed carrier form of modulation, or said another form ofmodulation is a suppressed carrier form of modulation, and said one formof modulation is a non-suppressed carrier form of modulation.
 12. Themethod of claim 2 where data packets are shorter than packets from voicesources.
 13. A cellular arrangement having mobile units and cells, whereeach cell covers a roughly circular geographical area by means of a basestation at the center of the cell, and where a plurality of cells arearranged to cover a larger geographical area, where each base stationcomprises: a receiver, a transmitter, and means for informing mobileunits of simultaneous detection by said receiver of packets in aninbound frequency channel, said informing being carried out by causingsaid transmitter to transmit, over a collision detection frequencychannel that is distinct from said inbound frequency channel and alsodistinct from outbound frequency channels that are used by saidtransmitter to send information packets to said mobile units.
 14. Thearrangement of claim 13 where the means for informing mobile units alsoinform the mobile units as to whether the inbound channel is busy. 15.The arrangement of claim 13 where said means for informing mobile unitscomprises: apparatus associated with the receiver which detectssimultaneous reception over the inbound frequency channel of datapackets and voice packets, and apparatus associated with the transmitterwhich signals a detection of said simultaneous reception of data packetsand voice packets over said collision detection frequency channel. 16.The arrangement of claim 13 where said means for informing mobile unitsincludes apparatus in the receiver that detects simultaneous receptionof packets.
 17. The arrangement of claim 16 where the apparatus thatdetects simultaneous reception of packets checks error correcting codesembedded in packets transmitted by mobile units.
 18. The arrangement ofclaim 13 where said transmitter assigns the frequency of the inboundchannel, and where the inbound channel and the outbound channel are in afirst frequency band that is substantially not overlapping withfrequency bands that are employed by transmitters of adjacent cells. 19.The arrangement of claim 18 where transmitters of adjacent cells employa second frequency band and a third frequency band such that adjacentcells use a different one of said second frequency band and thirdfrequency band.
 20. The arrangement of claim 13 where said transmitteralso transmits common control information over a frequency that isshared by transmitters of all of said cells, in a time divisionmultiplexing.
 21. The arrangement of claim 20 where the time divisionmultiplexing provides for three time channels, and a time channel usedby a base station is different from a time channel that is used by anadjacent base station.